[IR] Generify lowerings

#KT-1179

compiler/testData/codegen/box/valueClasses/classFlattening.kt

@@ -0,0 +1,21 @@
+// WITH_STDLIB
+// TARGET_BACKEND: JVM_IR
+// WORKS_WHEN_VALUE_CLASS
+// LANGUAGE: +ValueClasses
+
+@JvmInline
+value class IC(val x: UInt)
+
+fun ic(x: IC) = x.x
+
+@JvmInline
+value class SimpleMFVC(val x: UInt, val y: IC, val z: String)
+
+fun smfvc(ic: IC, x: SimpleMFVC, ic1: UInt) = ic(ic) + x.x + ic(x.y) + ic1
+
+@JvmInline
+value class GreaterMFVC(val x: SimpleMFVC, val y: IC, val z: SimpleMFVC)
+
+fun gmfvc(ic: IC, x: GreaterMFVC, ic1: UInt) = smfvc(ic, x.x, 0U) + ic(x.y) + smfvc(IC(0U), x.z, ic1)
+
+fun box() = "todo"